1. Field of the Invention
The present invention relates to a fuel injection control system for an internal combustion engine, which system is designed for performing a simultaneous fuel injection upon starting operation of the engine. More particularly, the present invention is concerned with a fuel injection control system for an internal combustion engine for ensuring a fuel quantity demanded for the simultaneous fuel injection while improving ignitability in the engine operation starting phase.
2. Description of Related Art
Heretofore, in the field of the internal combustion engines for motor vehicles such a fuel injection control system is known which is designed for performing a simultaneous fuel injection for a plurality of cylinders in every ignition cycle in an engine operation starting phase in which a battery voltage is likely to change remarkably with a view to enhancing the ignitability by stabilizing the fuel injection quantity.
For having better understanding of the invention, background techniques thereof will first be described in some detail. FIG. 7 is a functional block diagram showing schematically a hitherto known or conventional fuel injection control system for an internal combustion engine (hereinafter also referred to simply as the engine) which is presumed to be a three-cylinder engine, only by way of example. FIG. 8 is a timing chart for illustrating injector control operation performed by the conventional fuel injection control system in the engine operation starting phase (i.e., simultaneous fuel injection phase).
Referring to FIG. 8, a crank angle signal SGT carries a train of pulses each having a leading or rising edge and a trailing or falling edge indicative of reference positions (B75.degree. and B5.degree.), respectively, for each of cylinders #1 to #3.
Parenthetically, the reference position B75.degree. represents a position preceding by 75.degree. in terms of the crank angle or CA relative to the top dead center (TDC) in the compression stroke of each cylinder, while the reference position B5.degree. represents a position preceding by 5.degree. CA to the top dead center.
As is well known in the art, the reference position B75.degree. (leading or rising edge) is used as the reference position for the ordinary timer control performed on a cylinder-by-cylinder basis while the reference position B5.degree. (falling edge) is employed for setting the initial ignition timing on a cylinder-by-cylinder basis in the engine operation starting phase.
Further, fuel injection signals J1, J2 and J3 for the individual engine cylinders (#1, #2 and #3) are illustrated in FIG. 8 in combination with operation of an engine starting switch, a crank angle signal SGT and strokes (suction, compression, explosion, and exhaust strokes) of the individual cylinders.
Referring to FIG. 7, a crank angle sensor 1 is installed in association with the crank shaft of the engine (not shown) and designed to generate the crank angle signal SGT carrying a train of pulses which represent the reference positions (B75.degree., B5.degree.) for the individual cylinders (cylinder #1 to cylinder #3), respectively, in dependence on the rotational positions of the engine, as can be seen in FIG. 8.
A variety of sensors denoted generally and collectively by reference numeral 2 represent a water temperature sensor for detecting, for example, a temperature of cooling water as the temperature information of the engine in addition to a throttle opening sensor, an intake air flow sensor, an engine speed sensor, a starting switch sensor and the like, as is well known in the art. These sensors serve for generating a variety of information indicating operating states of the engine. Of course, the crank angle sensor 1 can also serve as the engine speed sensor as well and may thus be considered as one of the various sensors 2.
An electronic control unit (ECU) 3 which may be constituted by a microcomputer or microprocessor is so designed as to generate an ignition signal P and a fuel injection signal J for controlling the engine operation on the basis of the crank angle signal SGT derived from the output of the crank angle sensor 1 and the engine operating state information derived from the outputs of the various sensors 2.
An ignition system 4 is comprised of a power transistor (not shown), an ignition coil (not shown) and spark plugs (not shown either) and driven in response to the ignition signal P generated in synchronism with the crank angle signal SGT. The power transistor incorporated in the ignition system 4 is turned on and off in response to the ignition signal P. On the other hand, the ignition coil responds to the on/off operations of the power transistor by generating a high voltage for bringing about electric discharge at the spark plugs for driving the engine.
Each of the fuel injectors 5 is actuated in response to the fuel injection signal J (see FIG. 8) having a pulse width or duration substantially proportional to the engine load for injecting a predetermined amount or quantity of fuel into the-associated one of the cylinders of the engine. A basic pulse width or duration Tb of the fuel injection signal J corresponds to the fuel injection time duration of the injector and a sum of the fuel injection quantities in each ignition cycle represents the demanded fuel quantity mentioned previously.
The electronic control unit (ECU) 3 is comprised of a cylinder identifying means 31 for generating a cylinder identifying signal A, an ignition control means 32 for generating the ignition signal P and a fuel injection control means 33 for generating the fuel injection signal J.
The cylinder identifying means 31 incorporated in the electronic control unit 3 is designed to identify each of the engine cylinders on the basis of the crank angle signal SGT to thereby generate the cylinder identifying signal A.
On the other hand, the ignition control means 32 and the fuel injection control means 33 are designed to generate the ignition signal P and the fuel injection signal J, respectively, on the basis of the crank angle signal SGT, the engine operating states represented by the various sensor signals and the cylinder identifying signal A.
For realizing the simultaneous fuel injection in the engine operation starting phase, the fuel injection control means 33 is designed to generate the fuel injection signals J simultaneously for the individual cylinders after the cylinder identification, i.e., after generation of the cylinder identifying signal A.
In that case, the basic fuel quantity Fb per injection for each of the cylinders is set in dependence on the demanded fuel quantity Fs mentioned previously and the number N of the cylinders in accordance with expression (1): EQU Fb=Fs/N (1)
Thus, the driving time duration (i.e., the basic pulse width or duration Tb) of the fuel injector 5 for each cylinder is so set that the condition given by above expression (1) can be satisfied. Obviously, in the case of the three-cylinder engine, the basic fuel quantity Fb per injection is one third of the demanded fuel quantity Fs.
At this juncture, it should be mentioned that the cylinder identifying means 31, the ignition control means 32 and the fuel injection control means 33 incorporated in the electronic control unit 3 may be implemented as a program or programs which can be executed by a microcomputer or microprocessor constituting a main part of the electronic control unit 3.
Next, referring to FIG. 8, description will be made in concrete of the operation of the conventional fuel injection control system for the internal combustion engine shown in FIG. 7.
At first, when the operator or driver closes the starting switch, the engine is forced to rotate by a starter motor (not shown). The crank angle sensor 1 produces the crank angle signal SGT in synchronism with the engine rotation, which signal SGT is then inputted to the electronic control unit (ECU) 3.
Since the pulse width of the crank angle signal SGT is offset only for the specific cylinder (e.g. cylinder #2), as is known in the art, the cylinder identifying means 31 installed in the electronic control unit 3 can identify discriminatively the individual cylinders by comparing sequentially the pulse widths of the crank angle signal SGT.
The cylinder identifying means 31 generates the cylinder identifying signal A at the time point (time point t0) at which the cylinder identification has been completed.
Thus, the fuel injection control means 33 starts the simultaneous fuel injection control by outputting the fuel injection signals J1, J2 and J3 each having the basic pulse width Tb for the fuel injectors 5 of all the cylinders in every control cycle of the engine cylinder.
Further, the ignition control means 32 performs a sequential ignition control in the order of the cylinder #1, the cylinder #3 and the cylinder #2 at every ignition time point or timing (i.e., t1, t2, t3, . . . ) of the individual engine cylinders, which time point corresponds to the termination of the compression stroke, as can be seen from the waveforms of the ignition signals P1, P2 and P3 illustrated in FIG. 8.
In that case, because the basic fuel injection quantity Fb determined on the basis of the basic pulse width Tb corresponds to one third of the demanded fuel quantity Fs, the incipient or initial explosion takes place in the engine cylinder at a time point when the basic fuel injection quantity Fb has been accumulated over three cycles for that engine cylinder.
More specifically, the initial explosion takes place in the engine cylinder for which the demanded fuel quantity Fs (=3.times.Fb) is available due to accumulation up to the initial or first suction stroke from the start of the simultaneous fuel injection control. In this conjunction, it is noted that the basic fuel injection quantity Fb in the compression stroke plays no role in the immediately succeeding ignition control but is accumulated additively as the fuel injection quantity for the ignition control in the succeeding cycle.
In the case of the example illustrated in FIG. 8, the fuel injection quantity for the cylinder #1 reaches at first the demanded fuel quantity Fs. Thus, the initial explosion starts at the time point (t4) for the ignition control for the cylinder #1. More specifically, the initial explosion starts at a time point (time point t4) in the forth ignition cycle from the start (time point t0) of the simultaneous fuel injection control.
In this manner, the engine driving operation is carried out not only by the starting motor but also by the engine itself over the period during which the starting switch is closed. The complete explosion state prevails at the time point at which the engine speed (rpm) has reached a predetermined revolution number e.g. about 600 rpm.
Thus, the operator or driver may open the starting switch for electrically deenergizing the starter motor at the time point at which the engine speed can be regarded as having reached a predetermined revolution number or engine speed (rpm) indicative of the complete explosion state. Thus, the driving state is sustained only by the engine itself without the aid of the starter motor. Further, at this time point, the fuel injection control means 33 changes over the simultaneous fuel injection control mode to the ordinary sequential fuel injection control mode.
As will now be appreciated from the foregoing, in the conventional fuel injection control system for the internal combustion engine, the fuel injection is performed for all the cylinders through the simultaneous fuel injection control in the engine operation starting phase regardless of the stroke statuses of the individual cylinders. Consequently, in the engine in which the injecting direction of the fuel injector 5 is so oriented as to point to the spark plug of the ignition system 4 from the standpoint of design, the fuel injected during the suction stroke is likely to be deposited on the spark plug at the electric discharge gap thereof, giving rise to a problem that the engine starting performance may be degraded due to incapability or failure of the electric discharge.
Further, attempt for suppressing such fuel deposition will then encounter a problem that the degree of freedom in design concerning the disposition of the fuel injector 5 and the peripheral structure of the intake manifold is restricted, making it practically difficult or even impossible to implement the peripheral structures satisfactorily in a miniaturized structure.
Besides, the fuel injection during the suction stroke means that a raw gas will be discharged when the fuel injection temporally overlaps with the opening of the exhaust valve, incurring a problem of environmental pollution.
Additionally, in the conventional fuel injection control system for the internal combustion engine, the fuel injection signal J1; J2; j3 of the predetermined constant basic pulse width Tb is employed for the simultaneous fuel injection control. Consequently, four ignition cycles at least will intervene between the time point t0 at which the cylinder identification has been completed and the initial explosion start time point t4, which means that a lot of time is taken for the start of the initial or incipient explosion, incurring degradation of the starting performance of the engine.
As an approach for avoiding the delay of the initial explosion mentioned above, it is conceivable to perform a preliminary fuel injection for each of the engine cylinders before the cylinder identification is completed. In that case, however, an excess fuel injection quantity state will likely to occur particularly when the operator or driver turns on and off the starting switch at a high frequency, which will unwontedly result in electric conduction of the electric discharge gap of the spark plug. In that case, the starting performance of the engine will be degraded due to the mis-ignition, to another disadvantage.
Of course, it is conceivable to perform the sequential fuel injection control from the time point at which the cylinder identification is completed. However, execution of the sequential fuel injection control in the starting phase in which the battery voltage is instable, as mentioned previously, will give rise to a problem that the starting performance of the engine is degraded due to instability of the fuel injection quantity and hence poor ignitability.